The Beltway is pretty thick.
9 to 10 inches of concrete, and 6 inches of asphalt:
When the I-495 Capital Beltway in Virginia was widened 1974-1977, they
did a great job. The original road was almost entirely reinforced
concrete pavement (except for about a mile). The new lanes (1 each way
east of I-95/I-395, 2 each way north of I-95/I-395) were added, and
identical design reinforced concrete pavement was used. It was 9 or 10
inches thick, on top of a deep aggregate base course. The full
shoulders, right and left, were given a base of 6" plain concrete
pavement. The original lanes had extensive concrete patching on spalled
and cracked joint areas. The entire roadway and shoulders were overlaid
with 4 inches of hot-mix asphalt base course. Finally, a 2 inch hot-mix
asphalt surface course was applied.
This is from this usenet post:
http://groups.google.com/group/misc.transport.road/msg/42fc64a6126682e0?dmode=source&hl=en